In U.S. Pat. No. 4,309,965 there is described a new wood fuel combustion system developed by Professor Richard C. Hill of the University of Maine at Orono and assigned to the Board of Trustees of the University of Maine. According to the system developed by Professor Hill and described in the patent application Ser. No. 75,815, burning of wood fuel for heating purposes is accomplished first by combustion of wood in a high temperature, e.g. 1200.degree. F.-2000.degree. F. (650.degree. C.-1100.degree. C.) refractory environment; second by delayed propagation of the flue gases in a continuing high temperature refractory insulating environment to assure complete combustion; and third only after completion of combustion, by extraction of heat from the end products of such combustion. This is accomplished by providing a substantially vertical feed primary combustion chamber having a refractory base portion forming the locus of combustion and a water jacket or water environment around the upper portion. Flue gases are drafted away from the base of the combustion chamber. The base draft and water jacket combine to confine the locus of combustion to the base of the chamber.
A flue gas delay propagation channel coupled to the draft outlet at the base of the combustion chamber is lined with refractory material and affords delayed propagation in a high temperature environment sufficient to insure substantially complete burning of the products of primary combustion. A heat exchanger coupled to the output of the flue gas delay channel receives the hot gaseous end products of combustion and transfers heat from the gases to water or other fluid medium.
Actively induced draft in addition to natural draft may be provided. In addition to actively inducing the draft, air may be forced under pressure into the combustion chamber base portion to effect turbulent mixing of air and combustion gases. A turbulent mixture therefore follows the draft from the base of the combustion chamber through the flue gas delay channel where the travel time in a high temperature environment permits substantially complete secondary burning of the flue gases prior to heat exchange.
The water jacket arrangement is a coacting element of the water system cooperating with the heat exchanger downstream. However the primary purpose of the water jacket arrangement is for confinement of combustion and controlled burning in the primary combustion chamber. As disclosed in Ser. No. 75,815, the water jacket and heat exchanger therefore comprise separate elements of the boiler system requiring separate additional space.
An improvement upon this basic Hill invention is described in U.S. Pat. No. 4,366,805 filed Apr. 24, 1981 and entitled "Sector Controlled Wood-Type Fuel Burning Furnace". This patent application describes a furnace system incorporating the features of the basic invention but which permits controlled combustion of selected sectors or segments of the wood-type fuels or other solid fuels for a high turn down ratio. The primary combustion chamber comprises a refractory base portion for high temperature combustion and an upper portion with a water jacket for quenching combustion in the upper portions of the fuel. In a preferred form the invention is embodied in a generally cylindrical or radial configuration with a plurality of refractory material secondary burning delay channels leading radially away from the refractory base portion of the primary combustion chamber. The secondary channels conduct flue gases from different sectors of the fuel to separate heat exchange pipes or fire tubes passing through the cylindrical water jacket. Separate dampers are provided for separately controlling the draft through respective radially directed secondary channels and corresponding heat exchange tubes. This permits separate control over combustion of different sectors of the fuel. The rate of combustion and heat output of the furnace may be varied and controlled without sacrificing the efficiency, completeness, and intensity of combustion. The radial configuration also affords a compact geometry in which the water jacket and heat exchange elements occupy substantially the same space.
The primary objective of this sector control improvement however, is to afford a high turn down ratio, and this is accomplished by including a duplication of elements around the furnace for separate control of combustion of the fuel in small sectors at a time.